1. Field of the Invention
The present invention relates to a method and an arrangement for transfer between a cordless telecommunication system, such as DECT, and a background cellular mobile telecommunication system such as GSM. The invention is in particular conceived to be used when different operators operate the two systems. In a case where both systems are operated by the same operator and the DECT system is connected directly to the background GSM network, transfer can be effected in a more effective manner.
Generally, the present invention reduces the transfer time between the different systems by among other things transmitting transfer parameters to the portable terminal and setting up a preparatory three-party connection.
2. Discussion of the Background
A user requirement of transfer is that the process is to be on the whole imperceptible for the user. That is to say, the transfer is to take place rapidly and without inconvenient background noise so that the transition which the transfer inevitably implies does not become annoying. It should therefore be possible to carry out the transfer in approximately 1 second, preferably even more rapidly. Within the field, there is a number of documents which describe different solutions to this problem.
U.S. Pat. No. 5,260,988, WO 93/16549, U.S. Pat. No. 5,127,042 and U.S. Pat. No. 4,989,230 describe communication systems which consist of both cordless telephony and a cellular mobile telephone system. The system includes portable cordless units which can initiate and receive calls in both the cordless and the cellular system. Calls can be connected automatically to the system in which the subscriber is located. There is also the possibility of automatically connecting a call in progress between the different systems if the subscriber moves between systems. It is possible for the user to select manually a given system by a button procedure, either permanently or at a given time. On transfer between the two systems, a three-party call is utilized.
U.S. Pat. No. 5,235,632 describes a mobile telephone system which consists of an external system which consists of base stations, divided into cells, with high transmitting power connected to a mobile telephone exchange and an indoor system consisting of base stations with low transmitting power connected to a mobile telephone exchange. According to the document, the possibility exists of connecting a call between the internal and the external system by, for example, measuring signal strength from each system.
U.S. Pat. No. 5,309,502 relates to a radio telephone which combines the function of a cordless telephone and a cellular mobile telephone. The possibility exists, on connecting calls, of automatically selecting via which the communication is to take place. Nothing is mentioned about transfer during calls in progress.
U.S. Pat. No. 5,210,785 relates to a cordless communication system and a terminal which combines two different systems such as cordless telephony and cellular mobile telephony. According to the document, selection of the communication method is to take place automatically.
U.S. Pat. No. 5,329,574 describes a method of continuously maintaining telephone communication when a radio communication unit moves between two different communication systems. When the unit discovers that the communication quality is deteriorating in a first system, a connection packet is transmitted via a central control unit to a second system. The communication in the first system is maintained until the second system confirms it has taken over of the communication.
U.S. Pat. No. 5,212,684 relates to a communication system which utilizes portable radio telephones. The system consists of base stations and portable telephone units in accordance with the GSM standard. The portable telephone units can also work in accordance with the DECT standard. The system carries out both internal and external transfer but does not mention transfer between DECT and GSM.
WO91/16772 describes a method of transfer in a mobile radio communication system. According to the document, the mobile measures signal strengths of surrounding base stations. When the transfer is felt to be justified, the signal strength vectors are correlated with stored vectors and, if there is correlation, transfer takes place in a manner which depends on the position.
EP A1 615 392 describes a further example of transfer in a mobile radio system which utilizes the position of the mobile.
U.S. Pat. No. 5,345,499 relates to a method of transfer in a cellular radio system. The system consists of microcells and macrocells. In the system, the possibility exists of making connection and transfer dependent on the speed of the mobile. Thus, a mobile with a high speed can be prevented from connecting itself to a microcell if it exceeds a given threshold speed.
U.S. Pat. No. 5,276,906 shows a cellular radio telephone system which has two threshold levels for transfer. When the signal strength in a first cell falls below a first threshold value, transfer is initiated to a second cell by this cell being selected with the aid of the mobile. When the signal strength in the first cell then falls below a second threshold value, transfer to the second cell is carried out.